1. Field of the Invention
The present invention relates to self-propelled projectiles, and, more specifically, to hybrid (e.g., solid-liquid) propulsion systems for rockets and the like.
2. Description of the Related Art
Rocket motors generally fall into two classes: solid propellant motors in which a solid fuel element undergoes combustion to produce thrust that propels the rocket, and liquid propellant motors that accomplish the same function with a liquid fuel material. A hybrid rocket motor may be characterized as a cross between a solid propellant motor and a liquid propellant motor. Generally, hybrid motors use a fluid oxidizer to burn a solid fuel element; however, they may use a combustible liquid fuel and a solid oxidizer. The hybrid rocket propellant can be ignited by an igniter, such as an electrically-generated spark, by pyrotechnic means, or by initial injection of an ignition fluid which exothermically reacts with the liquid oxidizer.
Some of the more well-known advantages of a hybrid rocket motor over a purely solid fuel motor are the complete separation of fuel from the principal oxidizer, thus eliminating the potential for inadvertent ignition or catastrophic failure; the ability to optimize the combination of propellant ingredients regardless of whether they are solid or liquid; and the ability to easily start, stop, and restart the motor, thereby making the motor easily throttleable since the solid fuel component need not contain any oxidizer. Because of these features, the motor is easily mass produced under less hazardous conditions and at a smaller cost.
A conventional hybrid rocket motor includes a hollow housing or combustion chamber in which an elongated solid fuel component, or "grain" is secured. The liquid or gaseous oxidizer is provided in a tank or container mounted forward of the fuel grain and flows along the fuel grain. Ignition causes combustion of the fuel-oxidizer mixture at the exposed surface of the fuel grain, resulting in the generation of thrust as the high pressure combustion products are discharged through the rocket nozzle.
U.S. Pat. No. 5,099,645 to Schuler et al. discloses a hybrid rocket motor in which liquid oxidizer in a tank is passed through a heat exchanger to convert it to gaseous form and then used to oxidize a solid fuel grain. The gaseous oxidizer also is fed back to the tank to pressurize the remaining liquid oxidizer, and the solid grain burn is supplemented with additional liquid oxidizer. While this system provides acceptable results, it requires an unduly complicated, failure-prone delivery mechanism to ensure proper operation.
U.S. Pat. No. 5,010,730 to Knuth et al. discloses a hybrid rocket system which also converts the liquid oxidizer to gaseous form before presenting the oxidizer to the solid grain. In this system, the grain-oxidizer combustion products are passed to a secondary combustion chamber where, like the Schuler et al. system, the combustion process is supplemented with additional liquid oxidizer. Somewhat alike in concept, these two systems suffer from similar disadvantages as noted above.
U.S. Pat. No. 5,101,623 to Briley discloses a hybrid rocket motor in which a hollow tube extends from the liquid oxidizer tank into the interior of the combustion chamber holding the solid grain. At its nozzle end, the tube has a plurality of perforations. The perforations permit liquid oxidizer to be injected into the combustion chamber and react with the fuel grain in a relatively constant and uniform pattern. This design, however, does not address the larger issues of motor design.
U.S. Pat. No. 5,119,627 to Bradford et al. discloses a hybrid system in which the solid fuel grain has a tank of non-flammable pressurized gas disposed therein. The pressurized gas is fed to the liquid oxidizer tank and is used to force the oxidizer into the combustion chamber. The Bradford et al. system is a rather specialized design which, due to its relative complexity, has limited flexibility in practical applications.
Thus, while the prior art designs implement a variety of workable systems, they all entail fairly complicated delivery systems. This results in limited flexibility, increased fabrication costs, and higher failure rates.